1. Field of the Invention
The present invention relates to a toroidal-type continuously variable transmission for an automobile and a method for manufacturing a trunnion for use in the present toroidal-type continuously variable transmission.
2. Description of the Related Art
Conventionally, a toroidal-type continuously variable transmission is known, for example, in Japanese Utility Model Unexamined Publication No. 62-71465 of Showa. That is, as shown in FIGS. 4 and 5, on a shaft 1, there are rotatably supported an input disk 2 and an output disk 3 through needle shafts 4. On the back surface side of the input disk 2, a cam plate 5 is spline engaged with the shaft 1, a plurality of rollers 6 are interposed between the cam plate 5 and input disk 2, and there is disposed a pressure mechanism 7 of a loading cam type which is used to press the input disk 2 toward the output disk 3 side. The output disk 3 is engaged with an output gear 9 through a key 8, so that the output disk 3 and output gear 9 are able to rotate synchronously with each other.
Between the input disk 2 and output disk 3, there is interposed a trunnion 11 which is able to swing about a pivot shaft portion 10 and, in the central portion of the trunnion 11, there is disposed a displacement shaft 12. And, on the displacement shaft 12, there is rotatably supported a power roller 13. This power roller 13 includes a traction portion in contact with the input disk 2 and output disk 3, while the power roller 13 is rollably contacted in such a manner that it can incline on and between the input disk 2 and output disk 3.
Also, between the trunnion 11 and power roller 13, there is disposed a power roller bearing 14. This power roller bearing 14, while carrying a load applied to the power roller 13 in its thrust direction, allows the rotation of the power roller 13. In the power roller bearing 14, a plurality of balls 15 are held by an annular retainer 17 which is interposed between an annular outer race 16 disposed on the trunnion 11 side and the power roller 13 serving as a rotary part.
Further, the trunnion 11 comprises a main body plane portion 18 and the above-mentioned pivot shaft portions 10 respectively formed integrally with the two end portions of the main body plane portion 18; and, in the main body plane portion 18, there is formed a circular hole 19. In the circular hole 19, there is disposed a needle bearing 20, so that the displacement shaft 12 is supported in a freely rotatable manner. Also, the two pivot shaft portions 10 respectively disposed on the two end sides of the trunnion 11 are respectively supported on a support plate 21 in such a manner that they can be swung with respect to the support plate 21, whereby the inclination angle of the displacement shaft 12 can be freely adjusted by the swing motion of the trunnion 11.
Further, to one end portion of each of the trunnions 11, there is connected a drive rod 22; on the outer peripheral surface of the middle portion of the drive rod 22, there is disposed a drive piston 23; and, a drive cylinder 24 is inserted into the drive piston 23.
According to the above-structured toroidal-type continuously variable transmission, rotation transmitted to the cam plate 5 of the pressure device 7 from a drive source such as an engine is transmitted through the rollers 6 to the input disk 2. The rotation of the input disk 2 is transmitted through the power roller 13 to the output disk 3, while the rotation of the output disk 3 is taken out from the output gear 9.
To change a rotation speed ratio between an input side and an output side, a pair of drive pistons 23 may be shifted in the mutually opposite directions. With such shifts of the drive pistons 23, the trunnions 11 are also respectively shifted in the mutually opposite directions. This changes the direction of the tangential-direction force that acts on the contact portions between the peripheral surfaces 13a of the power rollers 13 and the inner peripheral surfaces 2a, 3a of the input and output disks 2, 3; and, with such change in the direction of this force, the trunnions 11 are respectively swung in the mutually opposite directions about their associated pivot shaft portions 10 pivotally supported by the support plate 21.
By the way, as shown in FIG. 6, the conventional trunnion 11 is worked by cutting a blank material consisting of a round rod. And, in case the trunnion 11 is experimentally assembled into a toroidal-type continuously variable transmission and a large torque is repeatedly input to the toroidal-type continuously variable transmission, an excessive load is input to the power roller 13, with the result that an excessive load is repeatedly applied to the trunnion 11 backing up the power roller 13.
On the other hand, since the toroidal-type continuously variable transmission is carried on board a car, the trunnion 11 must also have the necessary and minimum dimension. Therefore, due to the large load repeatedly applied, the center portion of the main body plane portion 18 of the trunnion 11, while supporting the two ends of the pivot shaft portions 10, is bent repeatedly. That is, when the displacement is large, there is applied a force of 4t or more when the maximum load of the engine of the car is applied to the trunnion 11 in the maximum deceleration, so that there is applied to the trunnion 11 such a large bending stress as shown by a broken line in FIG. 6.
As a result of this, the connecting portions A between the pivot shaft portions 10 and main body plane portion 18, in which the stress becomes the highest, is pulled and returned back to their original conditions repeatedly; that is, the bending stress is applied collectively to the connecting portions A, so that the connecting portions A are finally cracked and broken.
The reason for this is believed that the conventional trunnion 11 is formed by cutting a round rod and thus, as shown in FIG. 7, metal flows 11a at the high-stress portions of the trunnion 11 are cut off. Therefore, the present inventors have conducted another test in which a trunnion 11 is formed by cutting a flat plate instead of the round rod. However, in this test as well, the results are similar to those in the former test.
The present invention aims at eliminating the drawbacks found in the conventional toroidal-type continuously variable transmission. Accordingly, it is an object of the invention to provide a toroidal-type continuously variable transmission which can prevent a trunnion against damage even when an excessive load is repeatedly applied thereto to thereby be able to enhance the durability of the trunnion and thus the toroidal-type continuously variable transmission.
In attaining the above object, according to the invention, there is provided a toroidal-type continuously variable transmission comprising: an input disk; an output disk disposed coaxially with the input disk; a trunnion capable of swinging about its pivot shaft portion situated at a torsional relation with respect to the respective center axes of the input and output disks; and a power roller supported on the trunnion and inclinably rollable on and between the input and output disks, wherein the trunnion comprises a main body plane portion for holding a displacement shaft supporting the power roller rotatably and a pair of pivot shaft portions respectively formed in the two end portions of the main body plane portion integrally therewith, while two connecting portions respectively between the main body plane portion and the pair of pivot shaft portions are respectively formed so as to include continuous metal flows. Here, xe2x80x9ctorsional relationxe2x80x9d of the pivot shaft portion means a physical relation that the pivot shaft portion is disposed at position along an imaginary plane that is perpendicular to an imaginary line connecting the respective center axes of the input and output disks and distanced from the intersection of the imaginary plane and imaginary line.
Also, according to another aspect of the invention, there is provided a method for manufacturing the trunnion, wherein a blank material of the trunnion is disposed in upper and lower forging dies in such a manner that the metal flows of the blank material extend in parallel to the axial direction of the trunnion and, after then, the blank material is pressed in a direction at right angles to the axial direction of the present blank material, thereby producing the trunnion by forging.
In the trunnion, generally, an excessive stress is concentrated on the connecting portions between the main body plane portion and two pivot shaft portions. However, according to the present structure, since the metal flows of the blank material are allowed to extend along the configuration of the connecting portions, the metal flows are prevented from being cut in the portion where the stress is concentrated, which not only can enhance the strength of the trunnion but also, even when loads are input repeatedly, can prevent the trunnion from cracking to thereby be able to enhance the durability of the trunnion and thus the toroidal-type continuously variable transmission using such trunnion.
Also, in another aspect of the invention, in order that the metal flows of the blank material are allowed to extend along the connecting portions, the trunnion is manufactured by forging. To manufacture the trunnion by forging, at first, the blank material is disposed in the upper and lower forging dies in such a manner that the metal flows of the blank material extend in parallel to the trunnion axis direction and, after then, the blank material is forged in a direction at right angles to the axial direction of the blank material, that is, in the tightening direction of the upper and lower forging dies, so that the metal flows are made to extend along the connecting portions between the trunnion main body plane portion and the trunnion pivot shaft portions, thereby being able to enhance the strength of the connecting portions.